I know this topic appeared earlier, but I still have got those issues when my Patrons try to unlock a post through a button.
I’ve checked and redirect URI is exactly the same in both places (with https:// and / at the end) and — as far as I know — my Wordpress is using only https (it uses Really Simple SSL plugin that should take care of that).

For what is worth, I recenlty changed the hosting provider and installed SSL certificate that wasn’t there before.

Where should I look for solution? I even created a new API client on Patreon’s site to check if something is wrong there. What other options do I have?